1. Field of the Invention
The present invention relates to a rotor-type pump suitable for a hydraulic pump which is used to produce the oil pressure required to circulate lubricant to automobile parts such as various moving engine parts or to deliver working fluid to power steering for instance, and specifically to techniques of a rotor-type pump substantially similar to a construction of a Wankel engine employing a rotor with three lobes each of which follows a peri-trochoidal or epi-trochoidal curved surface of a rotor housing in which the rotor turns.
2. Description of the Prior Art
As is generally known, there are several general types of oil pumps used in pressure-feed systems, for lubrication of an internal combustion engine or for delivery of working fluid to a power steering system which serves to produce a steering assistance. In one, the external gear-type pump uses a pair of meshing gears. In another, the plunger pump uses a plurality of plungers. In another, the vane pump uses a plurality of vanes radially slidably set in slots in the pump rotor. Also, the internal gear-type pump such as a trochoid pump or a gear-type oil pump with a pair of meshing gears and a crescent between the gears, is often used. To provide a higher-performance pump, it is desired to apply a basic construction of the Wankel engine (often called a four-stroke cycle rotary-combustion engine) to hydraulic pump. Hereinafter described briefly is the construction and operation of typical Wankel engines. The Wankel engine typically has a rotor housing, a pair of flat-faced side housings enclosing and sealing the rotor housing, a main shaft or a crankshaft (more accurately an eccentric shaft) with a rotor journal eccentric to the center line of the eccentric shaft (equal to the drive shaft), a substantially triangular rotor being rotatable eccentrically in the rotor housing and having three rotor lobes or apexes which are circumferentially equidistant spaced to each other on the outer periphery of the rotor and follow the epi-trochoidal curve of the inner face of the housing. An external gear (stationary gear) member is mounted in one (usually a rear side housing) of the side housings and has the bearing that supports one end of the eccentric shaft, while an internal gear is installed in the rotor. The rotor fits on the eccentric rotor journal, and the internal gear (the rotor gear) meshes with the stationary gear of the side housing and is guided by the stationary gear and revolves around the latter. The apex seals on the three lobes are in contact with and tightly fit against the inner face of the rotor housing to provide a tight seal, and thus provide three separate chambers between the respective two adjacent rotor lobes. With the internal gear (the rotor) revolving around the stationary gear of the side housing, these chambers increase and decrease in volume. This action is similar to the decrease and increase in volume in the cylinder of a reciprocating engine, as the piston moves up and down. Ordinarily, an exhaust port is formed in the rotor housing, whereas an intake port is formed in the rotor housing or in the side housing, so that the two ports are arranged parallel to each other in one curved side wall of the rotor housing. Also provided in the rotor housing is at least one spark plug (usually a pair of spark plugs are provided such that the plugs face the two ports). In case of a well-known one-rotor Wankel engine, the gear ratio between the rotor gear and the stationary gear is set at 1:3 so that the output shaft (i.e., the eccentric shaft) rotates three times every revolution of the rotor, and so that there are four stages (namely an intake stroke, a compression stroke, a power stroke and an exhaust stroke) with respect to each of three chambers defined between three rotor lobes, during one revolution of the rotor. In detail, the Wankel engine operates as follows. With rotation of the rotor after one of the rotor lobes has cleared the intake port, the space (the volumetric capacity of the chamber) between the one lobe (the leading lobe), the adjacent lobe (the trailing lobe) and the housing begins to increases to produce a partial vacuum and to cause the air-fuel mixture to enter. With a further rotation of the rotor, the space between the rotor and housing continues to increase. When the rotor reaches the point wherein the trailing lobe passes the intake port, the air-fuel mixture is sealed between the leading and trailing lobes, and then the mixture is compressed. When the mixture is nearing the maximum compression at the last stage of the compression (near TDC on the compression stroke), the spark plugs fire to ignite the mixture and thus the combustion takes place. At this stage, the hot burnt gases push the rotor to turn it further around. Thereafter, the hot gases continue to expand and the expand stroke continues until the leading lobe has cleared the exhaust port. The hot burnt gases begin to exhaust from the space between these adjacent lobes via the exhaust port and the exhaust stroke continues. Then the leading lobe has cleared the intake port again. In this manner, the four stages (complete series of actions) are repeatedly executed every revolution of the rotor. Such a conventional Wankel engine has been disclosed in Japanese Utility Model Provisional Publication No. 64-15726. As set forth above, in the Wankel engine, the complete series of actions take place between three pairs of rotor lobes. Three sets of actions which occur at the same time in the engine, provide a high engine performance. However, it is very difficult to apply the basic construction of the Wankel engine as previously noted to oil pumps used in pressure-feed lubricating systems of automotive engines, for the reasons set out below. In Wankel engine the fuel system mixes a fine spray of fuel (gasoline) with air to make a combustible and compressible air-fuel mixture and the compressible mixture is compressed on compression stroke and ignited and expanded on power stroke, whereas the oil pump is used to produce an increased pressure of incompressible working fluid. That is, the Wankel engine is applied to a compressible fluid (air-fuel mixture) and so designed to function as an internal combustion engine by way of compressing and expanding action of compressible air-fuel mixture (i.e., changes in volume in the combustion chamber). On the other hand, oil pumps must be applied to incompressible fluid such as lubricating oil for automotive moving or rotating parts or working fluid for a power steering device. Also, in the typical Wankel engines, one meshing pair (the stationary gear and the rotor gear) are provided to control the rotation of the rotor in such a proper manner that the rotor satisfactorily follows the epi-trochoidal curve of the inner face of the rotor housing and rotates eccentrically around the stationary gear. Generally, the stationary gear is mounted in the side housing, whereas the rotor gear is installed in the rotor, and thus such a guiding device composed of the stationary gear (external gear) and the rotor gear (internal gear) requires a high machining accuracy of the meshing pair. Also, such a conventional guiding device has a complicated structure and is generally constructed by many parts. If the conventional guiding device is applied to a rotor-type pump, the high accuracy of machining of the meshing gears and the complicated structure of the guiding device increase production costs of the rotor-type pump which utilizes a basic construction of the Wankel engine. Additionally, the stationary gear and the rotor gear meshing each other have required a comparatively great installing space in the housing, and thus there is a tendency that the entire size and weight of the rotor-type pump increase.